The present invention relates to optoelectronic devices, and more particularly, to edge-emitting (or stripe) semiconductor lasers.
With the advent of fiber optic communication systems has come the demand for high power, highly efficient and highly integrated laser devices. High power semiconductor lasers are increasingly required for telecommunications systems, predominantly as power sources for optical amplifiers, as Continuous Wave (CW) laser sources used with external modulators, and as directly modulated lasers. Deployment costs, system sizes and electrical power requirements all mandate compact, low cost efficient laser sources.
Traditional semiconductor laser devices use direct current injection into a semiconductor junction to achieve population inversion and laser output. Spatial mode structure is typically controlled through the use of narrow nominally linear regions (so-called stripes, ridges, or mesas) with suitable guiding properties to ensure single spatial mode operation. Electrically pumping the active region leads to power dissipation, which is managed through the thermal conductivity of the surrounding materials and the device heatsink. The electrical pumping level must increase as the required optical power output from the laser is increased, and the power dissipation in the active region then increases, increasing the temperature of the active region. The temperature sensitivity of the laser parameters leads to a saturating light vs. current characteristic, whereby the maximum power which can be achieved from the device is the xe2x80x9croll overxe2x80x9d power. Various methods are implemented to increase the roll over power of the existing devices: reducing the electrical and the thermal impedance through longer and wider cavities, which in effect reduces the dissipated power density, reducing the temperature increase through improved heatsinking, or reducing the temperature sensitivity by changing the materials from which the device is constructed. There is also an advantage in enlarging the optical mode within the structure, for example to reduce device degradation depending on optical intensity, but this can be at odds with the required electrical injection efficiency.
The above methods are all employed to varying effect, and with varying degrees of difficulty, but a market for yet higher power devices, or devices with more reliable beam quality, or smaller chip size still exists.
An alternative method of achieving high powers with good beam quality is to use electrically pumped diode lasers (generally large area, multi-mode devices) to optically pump a bulk optic laser material(such as Nd:YAG) which is configured for suitable beam quality. Furthermore, the optical pumping of a lasing material using laser diodes is the only viable option since it is impossible to inject current into certain lasing materials such as Nd:YAG. These so-called xe2x80x9cbrightness converterxe2x80x9d systems have the advantage over directly electrically pumped sources that the electrically pumped diode lasers are running at moderate power density and don""t need complex mode control, and that there is little power dissipation in the optically pumped laser material. However, brightness converters are not as efficient as the electrically pumped sources due to a double conversion from electrical to optical, then from optical to optical power.
Power dissipation may also be somewhat improved by reducing the electrical resistivity of laser cavity through increased doping of the p-type material, which is generally the predominant contributor to the laser cavity""s series resistance. However, increasing the p-doping level causes higher propagation loss, higher power dissipation in the cavity, lower optical output power and lower efficiency.
Accordingly, there is a strong need for a high power, low cost, small, efficient source which can be made with arbitrary output wavelength. Preferably, such a laser device has low power dissipation density and provides for effective delivery of optical power.
The above problems and other similar shortcomings of the existing semiconductor laser designs are solved by the novel use of monolithic optical pumping of an edge-emitting semiconductor laser. The invention makes use of the remote electrical power dissipation, tolerance to multi-mode behaviour in the pump source, and large optical cavity design independent of electrical injection efficiency issues associated with the optically pumped devices in order to achieve high output power, while providing the single chip semiconductor benefits of small size, high efficiency, and mechanical simplicity, to achieve the required flexibility and low cost over a wide range of output wavelength. The broad area cavity design provides high power pumping while preserving good spatial mode cavity characteristically associated with narrow stripe lasers.
The invention features a monolithically integrated optical source (a pump laser) to pump a second source (an edge-emitting signal laser), thereby allowing to reduce the heating of the active region of the signal laser by generating and removing the heat often produced in operating electrically pumped lasers a distance away from the signal laser active region. Furthermore, the pump laser has a broad area to minimize the dissipated power density and therefore reduce the heating in the pump laser.
The current invention arises from the realization that in most state-of-the art semiconductor lasers, only a fraction of the injected electrical energy is converted into laser light, and the remaining energy is dissipated within the laser structure as heat. As a result, high power laser output is severely limited by the thermal dissipation resulting from carrier flow during electrical excitation. These parasitic thermal effects can be obviated by photopumping the active region of the edge-emitting signal laser thereby minimizing excessive heating typically associated with current injection. The current invention departs from merely increasing the size of the laser device to improve thermal dissipation and instead focuses on optically pumping the active region of the edge emitting signal laser, thereby improving the optical output power of the edge-emitting signal laser without affecting the stability of the overall system.
Maintaining stable single mode operation is difficult to achieve in large area lasers. However, the pump laser proposed need not be a single mode source, as only the signal laser is required to produce single mode output required for efficient coupling to single mode fibers. The signal laser achieves single lateral and transverse mode operation through straightforward cavity design, as the laser does not need the very large cavities required by electrically pumped high power lasers.
According to one aspect of the current invention, optical reflectors are placed at the sides of the laser device in order to create a resonating optical cavity within the laser device.
According to another aspect of the invention, the signal laser may be configured and designed to operate as an optical amplifier by reducing the reflectivity of the optical reflectors to extremely low levels.
According to another aspect of the invention, a transition region is fabricated between the active region of the edge-emitting signal laser and the active region of the pump laser. The transition region serves as a waveguide to channel and deliver pump light generated by the pump laser to the active region of the edge-emitting signal laser.
According to another aspect of the invention, additional semiconductor pump lasers are monolithically integrated with the edge-emitting signal laser to provide further optical pumping of the active region of the edge-emitting signal laser, such that the rate of carrier recombination and power output of the laser device is generally substantially increased.
Another aspect of the present invention provides a method of fabricating an edge-emitting photopumped semiconductor laser comprising the steps of providing a substrate, fabricating thereon a pump laser active region as well as a signal laser active region. The fabrication process also involves forming a first reflective surface and a second reflective on a first and a second side wall of the laser structure, as well as providing means for excitation of the pump laser active region. Similar fabrication steps may be used to build an edge-emitting photopumped semiconductor laser having a plurality of pump lasers. The pump lasers may be arranged in one- or two-dimensional arrays.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.